Add automated tests for pyright-language-service

### Task Summary

`pyright-language-service` has no automated tests and no CI coverage. Dependency bumps there (e.g. #6176, express 4 → 5) currently require manual verification: starting the service and driving an LSP handshake over WebSocket by hand.

Add a smoke test that starts the service, connects a WebSocket client to `/python-language-server`, runs `initialize` → `didOpen` with a Python snippet containing known errors, and asserts the expected pyright diagnostics are published. Wire it into CI so future dependency PRs are verified automatically.

### Task Type

- [ ] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [x] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other